KR20220022472A - laser focusing module - Google Patents

Abstract

A laser focusing system 330 for use in an EUV radiation source is described, comprising: a first curved type configured to receive a laser beam from a beam delivery system 320 and generate a first reflected laser beam 316 . mirror 330.1; - a second curved mirror (330.2) configured to receive the first reflected laser beam (316) and generate a second reflected laser beam (317), the laser focusing system (330) comprising a second reflected laser beam (317) to a target location 340 within the vessel 350 of the EUV radiation source 360 .

Description

laser focusing module

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to EP application 19167261.7, filed on April 4, 2019, the contents of which are incorporated herein by reference in their entirety.

The present invention relates to a laser focusing system as applicable to EUV radiation sources. The invention also relates to an EUV radiation source comprising a laser focusing system.

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, typically onto a target portion of the substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In this case, a patterning device, alternatively referred to as a mask or reticle, may be used to create a circuit pattern to be formed on an individual layer of the IC. This pattern may be transferred onto a target portion (eg, comprising a portion of a die, one or several dies) on a substrate (eg, a silicon wafer). Transfer of the pattern is typically via imaging into a layer of radiation-sensitive material (resist) provided on a substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

Lithography is widely recognized as one of the key steps in the fabrication of ICs and other devices and/or structures. However, as features made using lithography become smaller in size, lithography is becoming an increasingly important factor for enabling miniature ICs or other devices and/or structures to be fabricated.

A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in Equation 1 below:

where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k ₁ is a process dependent adjustment factor, also called Rayleigh constant, and CD is the feature size (or criticality) of the printed feature. dimension). From Equation 1, it can be seen that the reduction of the minimum printable size of a feature can be achieved in three ways; It is concluded that it can be obtained by shortening the exposure wavelength λ, increasing the numerical aperture NA, or decreasing the value of k ₁ .

In order to shorten the exposure wavelength and thus reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5 to 20 nm, for example within the range of 13 to 14 nm. It has been further proposed that EUV radiation with a wavelength of less than 10 nm can be used, for example within the range of 5 to 10 nm, such as for example 6.7 nm or 6.8 nm. Possible sources include laser-generated plasma (LPP) sources, although other types of sources are possible.

An example of the current progress in the development of LPP sources for EUV lithography can be found in the paper "High power LPP EUV source system development status" by Benjamin Szu-Min Lin, David Brandt, and Nigel Farrar, SPIE Proceedings Vol. 7520, Lithography Asia 2009, December 2009 (SPIE Digital Library reference DOI: 10.1117/12.839488). In lithographic systems, the source apparatus will typically be contained within its own vacuum housing, whereas a small exit aperture is provided to couple the EUV radiation beam to the optical system in which the radiation will be used.

In order to be useful for high-resolution patterning for lithography, the EUV radiation beam must be tuned to obtain desired parameters such as uniformity of intensity and angular distribution when reaching the reticle. Examples of lighting systems are described in US Patent Application Publication Nos. US2005/0274897A1 (Carl Zeiss/ASML) and US2011/0063598A (Carl Zeiss). The exemplary system includes a "fly's eye" illuminator that converts the highly non-uniform intensity profile of the EUV source into a more uniform and controllable source.

To generate EUV radiation in an LPP radiation source, a laser or laser system is used to irradiate a target, such as a Sn droplet. In particular, such LPP radiation sources may include one or more lasers for irradiating the target with one or more pre pulses and one or more main pulses to divert the target and generate EUV radiation.

Generally, such lasers or laser systems include a laser focusing system configured to focus a laser beam, eg, a pre-pulsed laser beam or a main pulsed laser beam, to a target location inside a vessel of an LPP radiation source or EUV source. include Known arrangements of such laser focusing systems can be quite bulky and also produce a focused laser beam that does not have the desired optical properties to irradiate the target and thus effectively convert the target into EUV radiation.

Aspects of embodiments of the present invention aim to provide an alternative laser focusing system for use in EUV radiation sources.

According to an aspect of the present invention, there is provided a laser focusing system for use in an EUV radiation source, the laser focusing system comprising:

- a first curved mirror configured to receive the laser beam from the beam delivery system and generate a first reflected laser beam;

- a second curved mirror configured to receive the first reflected laser beam and generate a second reflected laser beam,

wherein the laser focusing system is configured to focus the second reflected laser beam at a target location within the vessel of the EUV radiation source.

According to another aspect of the present invention, there is provided a laser source comprising a laser focusing system according to the present invention.

According to another aspect of the invention, there is provided an EUV radiation source comprising a laser source according to the invention.

According to another aspect of the present invention, there is provided a lithographic apparatus comprising an EUV radiation source according to the present invention.

These aspects of the invention and their various optional features and implementations will be understood by those skilled in the art from the description of the examples that follow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference signs indicate corresponding parts:
1 schematically illustrates a lithographic system according to an embodiment of the present invention.
Figure 2 shows a more detailed view of the device of Figure 1;
3 shows a laser system comprising a laser focusing system according to the present invention.
4 shows a laser focusing system as known in the art.
5 shows a laser focusing system according to an embodiment of the present invention.
6 shows a fuel target as applied to an FPP radiation source and a laser beam configured to impinge on the fuel target.
7 shows a laser focusing system according to the invention in which a first control strategy is implemented.
8 shows a laser focusing system according to the invention in which a second control strategy is implemented.
9 shows a laser focusing system according to the invention in which a third control strategy is implemented.
10 to 12 show three possible embodiments of a laser focusing system according to the invention.

1 schematically shows a lithographic system 100 according to an embodiment of the present invention, the lithographic system comprising a lithographic apparatus and an EUV radiation source configured to generate an EUV radiation beam, for example an EUV radiation beam . In the embodiment as shown, the EVU radiation source comprises a source collector module. In an embodiment as shown, the lithographic scanning apparatus comprises an illumination system (illuminator) (IL) configured to modulate a radiation beam B (eg, UV radiation); A support structure (eg, mask table) (MT) configured to support a patterning device (eg, mask, or reticle) MA and coupled to a first positioner PM configured to accurately position the patterning device ); A substrate table (eg, a wafer table) configured to hold a substrate (eg, a resist-coated wafer) W and coupled to a second positioner PW configured to accurately position the substrate (WT) ); A projection system (eg comprising one or more dies) configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (eg comprising one or more dies) of the substrate W (eg , a refractive projection lens system) (PS).

The illumination system IL can be used to direct, shape, or control radiation of various types, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof. It may include an optical component.

The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions such as, for example, whether the patterning device is maintained in a vacuum environment. The support structure may utilize mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be, for example, a frame or table, which may be fixed or movable as required. The support structure may ensure that the patterning device is in a desired position, for example relative to the projection system.

The term “patterning device” should be interpreted broadly to refer to any device that can be used to impart a pattern in the cross-section of a beam of radiation, such as for creating a pattern in a target portion of a substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array uses a matrix arrangement of small mirrors, each of which can be individually tilted to reflect an incoming radiation beam in different directions. The tilted mirror imparts a pattern to the radiation beam reflected by the mirror matrix.

Like the illumination system, the projection system may contain refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical elements, or any of these, suitable for the exposure radiation being used or for other factors such as the use of a vacuum. may include various types of optical components, such as any combination of It may be desirable to use a vacuum for EUV radiation because other gases may absorb too much radiation. A vacuum environment can thus be provided in the entire beam path with the aid of the vacuum wall and vacuum pump.

As shown in the figure, the apparatus may be of a reflective type (eg, using a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multi-stage” machines, additional tables may be used simultaneously, or preparatory steps may be performed on one or more tables while one or more other tables are being used for exposure.

Referring to FIG. 1 , an illuminator IL receives a beam of extreme ultraviolet radiation from a source collector module SO of an EUV radiation source. A method for generating EUV light includes, but is not necessarily limited to, converting a material to a plasma state having at least one element, such as xenon, lithium, or tin, with one or more emission lines within the EUV range. In one such method, a desired plasma, often referred to as a laser generated plasma (“LPP”), may be generated by irradiating a fuel, such as droplets, stream, or clusters of material, with a desired line emitting element with a laser beam. The source collector module SO may be part of an EUV radiation system comprising a laser, not shown in FIG. 1 , for providing a laser beam to excite the fuel. The resulting plasma emits radiation, eg EUV radiation, which is collected using a radiation collector disposed in a source collector module. The laser and EUV radiation source may be separate entities, for example when a CO ₂ laser is used to provide a laser beam for fuel excitation.

In this case, the laser is not considered to form part of the lithography system, and the radiation beam is directed from the laser to the source collector with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or beam expanders. Go to the mirror. In other cases, for example, when the source is a discharge generating plasma EUV generator often referred to as a DPP source, the source may be an integral part of the source collector module.

The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. In general, at least the outer and/or inner radial extent of the intensity distribution within the pupil plane of the illuminator (generally referred to as “σ-outer” and “σ-inner”, respectively) can be adjusted. The illuminator IL may also include various other components such as facet fields and pupil mirror devices. The illuminator may be used to steer the radiation beam to have a desired uniformity and intensity distribution in the cross-section of the radiation beam.

The radiation beam B is incident on a patterning device (eg mask) MA held on a support structure (eg mask table) MT and is patterned by the patterning device. After being reflected off the patterning device (eg mask) MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of the substrate W. make it With the aid of the second positioner PW and the position sensor PS2 (for example an interferometric device, a linear encoder or a capacitive sensor) different target parts C, for example, in the path of the radiation beam B For positioning, the substrate table WT can be accurately moved. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (eg mask) MA with respect to the path of the radiation beam B. Patterning device (eg, mask) MA and substrate W may be aligned using mask alignment marks M1 , M2 and substrate alignment marks P1 , P2 .

The device shown can be used in at least one of the following modes:

1. In step mode, the support structure (eg mask table) MT and substrate table WT remain essentially fixed, while the entire pattern imparted to the radiation beam B is applied at once It is projected onto the target portion C (ie a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that different target portions C can be exposed.

2. In the scan mode, the support structure (eg mask table) MT and the substrate table WT are scanned simultaneously while the pattern imparted to the radiation beam is projected onto the target portion C (ie, a single dynamic exposure). The speed and direction of the substrate table WT relative to the support structure (eg, mask table) MT may be determined by the magnification (reduction) and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (eg mask table) MT remains essentially fixed to hold the programmable patterning device, wherein a pattern imparted to the radiation beam is applied onto the target portion C. The substrate table WT is moved or scanned while being projected onto the . In this mode, typically a pulsed radiation source is used and the programmable patterning device is updated as needed after each movement of the substrate table WT, or between successive radiation pulses during scanning. This mode of operation can be readily applied to maskless lithography using a programmable patterning device, such as a programmable mirror array of a type as referred to above.

Combinations and/or variations on the modes of use described above or completely different modes of use may also be employed. The embodiment to be illustrated includes scanning as in modes 2 and 3 just mentioned.

Although specific reference may be made herein to the use of a lithographic apparatus in the manufacture of ICs, the lithographic apparatus described herein may include integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid crystal displays ( It should be understood that it may have other applications such as manufacturing of LCDs), thin film magnetic heads, and the like. The skilled person will appreciate that, in the context of these alternative applications, any use of the terms “wafer” or “die” within this specification may be considered synonymous with the more general terms “substrate” or “target portion”, respectively. will recognize The substrates referred to herein may be processed before or after exposure, for example in a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), or metrology tools and/or inspection tools. Where applicable, the invention herein may be applied to these and other substrate processing tools. Also, a substrate may be processed more than once, for example to create a multilayer IC, and thus the term substrate as used herein may also refer to a substrate that already contains multiple processed layers.

2 shows in more detail a system 100 comprising an EUV radiation source comprising a source collector module SO and a lithographic scanning apparatus comprising an illumination system IL and a projection system PS. The source collector module SO of the EUV radiation source is constructed and arranged such that a vacuum environment can be maintained within the enclosure structure 220 of the source collector module SO. The systems (IL and PS) are likewise contained in their own vacuum environment. EUV radiation emitting plasma 210 may be formed by a laser generated LPP plasma source. The function of the source collector module SO is to deliver the EUV radiation beam 20 from the plasma 210 such that the EUV radiation beam is focused on a virtual source point. The virtual source point is generally referred to as an intermediate focal point IF, and the source collector module is arranged such that the intermediate focal point IF is located at or near the opening 221 of the enclosure structure 220 . The virtual source point IF is an image of the radiation emitting plasma 210 .

From the aperture 221 at the intermediate focus IF, the radiation traverses the illumination system IL, which in this example comprises a facetted field mirror device 22 and a facetted pupil mirror device. (24) is included. These devices form a so-called "fly's eye" illuminator, which illuminator not only the desired angular distribution of the radiation beam 21 in the patterning device MA, but also the desired uniformity of the radiation intensity in the patterning apparatus MA. arranged to provide sex. Upon reflection of the beam 21 at the patterning device MA held by the support structure (mask table) MT, a patterned beam 26 is formed and the patterned beam 26 is directed to the projection system PS. is imaged, via reflective elements 28 and 30, onto a substrate W held by a wafer stage or substrate table WT.

Each system IL and PS is arranged in its own vacuum or near-vacuum environment defined by an enclosure structure similar to enclosure structure 220 . There may be more elements present in the illumination system IL and the projection system PS than are shown generally. Also, there may be more mirrors than shown in the figures. For example, in addition to the reflective elements shown in FIG. 2 , there may be 1 to 6 additional reflective elements present in the illumination system IL and/or the projection system PS. The above-mentioned US patent application publication, for example, shows three additional elements in a lighting system.

Considering the source collector module (SO) in more detail, a laser energy source comprising a laser 223 is configured to deposit laser energy 224 onto a fuel such as xenon (Xe), tin (Sn) or lithium (Li). arranged to create a highly ionized plasma 210 having an electron temperature of several tens of eV. Higher energy EVU radiation can be produced with other fuel materials such as, for example, Tb and Gd. Energy radiation generated during the de-excitation and recombination of these ions is emitted from the plasma, is collected by a near normal incidence collector (CO) and focused on an opening 221 . The plasma 210 and the opening 221 are located at the first and second focal points of the collector or collector mirror CO, respectively.

To deliver fuel, for example liquid tin, a droplet generator 226 is arranged within the enclosure structure 220 and is arranged to fire a high frequency stream 228 of droplets towards a desired location in the plasma 210 . . In operation, laser energy 224 is delivered concurrently with operation of droplet generator 226 , delivering an impulse of radiation to convert each fuel droplet into plasma 210 . The propagation frequency of the droplet may be several kilohertz, for example 50 kHz. In practice, laser energy 224 is delivered in at least two pulses: to vaporize the fuel material into a small cloud, a pre pulse with limited energy is delivered to the droplet before reaching the plasma location, then the energy A main pulse of 224 is delivered to the cloud at a desired location to generate plasma 210 . A trap 230 is provided on the opposite side of the enclosure structure 220 to capture fuel that does not turn into a plasma for any reason.

Although not illustrated herein, the source collector module and many additional components within the lithographic apparatus are present in a typical apparatus. They contain arrangements to reduce or mitigate the effects of contamination within an enclosed vacuum, preventing, for example, deposits of fuel material from damaging or deteriorating the performance of collectors or collector mirrors (CO) and other optics. do. In addition, one or more spectral purity filters may be included in the source collector module SO and/or the illumination system IL. In addition to the desired wavelength of UV radiation, these filters are intended to remove as much as possible radiation of undesired wavelengths generated by the laser and/or plasma 210 . The spectral purity filter(s) may be located near the virtual source point or at any point between the collector and the virtual source point. The filter may be placed at another location in the radiation path, for example downstream of the virtual source point IF. A number of filters may be deployed. The skilled person is well aware of the need for these measures and the manner in which they may be implemented, and no further details are required for the purposes of the present invention.

Referring more closely to the laser 223 from FIG. 2 , the laser in the presented embodiment is of the MOPA (Master Oscillator Power Amplifier) type. It consists of a “master” laser or “seed” laser, denoted MO in the figure, followed by a power amplifier (PA). A beam delivery system 240 is provided to deliver laser energy 224 to the module SO. In practice, the pre-pulse component of the laser energy will be delivered by a separate laser not shown separately in the figures. Laser 223 , fuel source (ie, droplet generator) 226 , and other components may be controlled by, for example, source control module 242 .

As will be appreciated by those skilled in the art, the reference axes X, Y and Z may be defined for measuring and describing the geometry and behavior of the device, its various components, and the radiation beams 20, 21, 26. can For each part of the device, a local frame of reference in the X, Y and Z axes can be defined. The Z axis generally coincides with the direction of the optical axis O at a given point in the system and is generally perpendicular to the plane of the patterning device (reticle) MA and perpendicular to the plane of the substrate W. In the source collector module, the X axis generally coincides with the direction of the fuel stream 228 (described below), while the Y axis is orthogonal thereto and points out of the drawing as shown in FIG. 2 . On the other hand, in the vicinity of the support structure MT holding the reticle MA, the X axis generally crosses the scanning direction parallel to the Y axis. For convenience, in this area of the schematic diagram FIG. 2 , the X axis points out of the drawing as indicated again. This designation is common in the art and will be adopted herein for convenience. In principle, any frame of reference can be chosen to describe the device and its behavior.

Referring in more detail to the illumination system, the faceted field mirror device 22 comprises an array of individual facets, so that the EUV radiation beam 20 is split into a number of sub-beams, one of which is denoted 260 in the figure. . Each sub-beam is directed to a separate facet on the facet pupil mirror device 24 . The facets of the pupil mirror device 24 are arranged to direct their respective sub-beams onto a target, which is a slit-like area of the patterning device MA. The split into sub-beams 260 and the combination into a single beam 21 are designed to produce very uniform illumination across the slit area when the illumination arriving from the source collector module is very non-uniform in its angular distribution. As is also known, facets of devices 22 and/or 24 may be steerable and/or maskable to implement different illumination modes. The adjusted EUV radiation beam 21 is delivered to the patterning device MA via a steering and masking module 262 . This module contains a masking unit, also referred to as a reticle mask (REMA), which may have movable blades that define the extent of the illumination slit in the X and Y directions. Typically, the illumination slit as applied in EUV type lithographic apparatus may be curved. In front of the REMA may also be an illumination uniformity correction module (UNICOM).

To expose a target portion C on the substrate W, a pulse of radiation is generated on the substrate table WT and the mask table MT performs synchronized movements 266 and 268 to pattern through the slits of illumination. Scan the pattern on the device MA.

Examples of lighting systems including REMA and UNICOM functions are described in US Patent Application Publication Nos. 2005/0274897A1 and 2011/0063598 A.

A number of measures are applied to the source controller 242 . These measures include monitoring at the outlet of the source collector module SO to ensure that the virtual source point IF is aligned with the opening 221 . In systems based on LPP sources, control of alignment is achieved by controlling the position of plasma 210 rather than by moving collector optics CO throughout. The collector optics, the exit aperture 221 and the illuminator IL are precisely aligned during the set-up process, so that the aperture 221 is positioned at the second focal point of the collector optics. However, the exact position of the virtual source point IF formed by the EUV radiation at the exit of the source optics depends on the exact position of the plasma 210 with respect to the first focus of the collector optics. Overall, active monitoring and control is required to pinpoint this position accurately enough to maintain sufficient alignment.

For this purpose, the source control module (controller) 242 in this example controls the position of the plasma 210 (source of EUV radiation) by controlling the injection of fuel, and also energizing pulses, for example from a laser. Controls the timing of (energizing pulse). In a typical example, an energizing pulse of laser radiation 224 is delivered at a rate of 50 kHz (period of 20 μs), and lasts from 20 ms to 20 seconds in burst. The duration of each main laser pulse may be about 1 μs, while the resulting EUV radiation pulse may last about 2 μs. With appropriate control, it is maintained that the EUV radiation beam is accurately focused on the opening 221 by the collector mirror CO. If this is not achieved, all or part of the beam will impinge on the surrounding material of the enclosure structure.

The source control module 242 is fed monitoring data from an array of one or more sensors (not shown), the array of sensors providing a first feedback path for information regarding the location of the plasma. The sensor may be of various types as described, for example, in US Patent Application Publication 2005/0274897A1 referenced above. The sensors may be located at one or more locations along the radiation beam path. They may be located around and/or behind the field mirror device 22 , for example, purely for example purposes. The sensor signals just described can be used for control of the optical systems of the illuminator IL and the projection system PS. They may also be used to assist the control module 242 of the source collector module SO via a feedback path to adjust the intensity and position of the EUV plasma source 210 . The sensor signal may be processed, for example, to determine the observed position of the virtual source IF, which is extrapolated to indirectly determine the position of the EUV source. If the virtual source position drifts, as indicated by the sensor signal, a correction is applied by the control module 242 to re-center the beam at the aperture 221 .

Rather than relying entirely on signals from the illuminator sensor, additional sensors and feedback paths may be provided throughout the source collector module SO itself to provide faster, more direct and/or independent control of the radiation source. Such sensors may include, for example, one or more cameras that monitor the position of the plasma. In this way, the locating beam 20 is maintained within the aperture 221 , damage to the equipment is prevented, and an efficient use of the radiation is maintained.

EUV radiation systems as described above with reference to FIGS. 1 and 2 typically include a laser or laser system not shown in FIG. 1 to provide a laser beam that excites fuel, eg tin (Sn) droplets. something to do. Typically, a laser or laser system is configured to deliver laser energy in at least two pulses; To vaporize the fuel material into a small cloud, a pre-pulse with limited energy is delivered to the droplets before reaching the plasma location. A main pulse of laser energy is then delivered to the cloud at a desired location to create a plasma. The resulting plasma emits output radiation, eg EUV radiation, which is collected using a radiation collector disposed in a source collector module.

In general, the laser system and EUV radiation source may be separate entities, for example when a CO ₂ laser is used to provide a laser beam for fuel excitation.

In such an arrangement, the laser system comprises, for example, a laser beam or a laser source configured to generate a beam, an optional laser beam delivery system configured to guide or direct the generated laser beam from the laser towards the EUV radiation source, and a laser and a laser focusing system configured to focus the beam onto a target inside the vessel of the EUV radiation source, eg, a tin droplet.

The present invention provides a laser focusing system applicable to such a laser system.

3 schematically shows a laser system 300 , which includes a laser or laser source 310 configured to generate a laser beam 315 . This laser or laser source 310 may include, for example, a seed laser and one or more power amplifiers for generating the laser beam 315 . The laser system 300 further includes a beam delivery system 320 configured to guide or direct the laser beam 315 towards a desired location. In the embodiment as shown, the laser system 300 further comprises a laser focusing system 330 according to an embodiment of the present invention. It is noted that the laser focusing system 330 may also serve to guide or direct the laser beam towards a desired location, as will be described in more detail below.

In the embodiment as shown, the laser focusing system 330 is a first curved mirror 330.1 configured to receive the laser beam 315 from the beam delivery system 320 and to generate a first reflected laser beam 316 . ) is included. The laser focusing system 330 further includes a second curved mirror 330.2 configured to receive the first reflected laser beam 316 and to generate a second reflected laser beam 317 . In the embodiment as shown, laser focusing system 330 is configured to focus second reflected laser beam 317 onto target location 340 . 3 schematically shows the target location 340 as being arranged inside a vessel or enclosure 350 of an EUV radiation source or source collector module 360 . Accordingly, this enclosure 350 can be compared with the enclosure 220 of the source collector module SO as shown in FIG. 2 .

In contrast to the arrangement shown in FIG. 2 , the laser focusing system 330 according to an embodiment of the present invention is configured in such a way that the second reflected laser beam 317 has an optical axis 370 substantially parallel to a horizontal plane. are arranged. In an embodiment of the invention, the laser focusing system is thus configured in such a way that the laser beam 317 focused on the droplet target in the EUV radiation source has an optical axis that is substantially horizontal, ie substantially parallel to the indicated Y-direction. Substantially horizontal within the meaning of the present invention denotes an angle with respect to the horizontal plane of 25 degrees or less, preferably less than 20 degrees. Application of a laser focusing system, such as system 330 , enables arranging the second reflected laser beam 317 substantially parallel to the laser beam 315 as received from the beam delivery system. Within the meaning of the present invention, two laser beams may be configured to be substantially parallel when the difference in orientation between the laser beams is less than 25 degrees, preferably less than 20 degrees. In the case where the laser beam 315 has a substantially horizontal orientation, the optical axis of the laser beam 317 focused on the droplet target is thus at an angle greater than 65 degrees to the vertical axis, ie the indicated Z-axis. As will be appreciated by those skilled in the art, the choice of the angle of the optical axis of the laser beam 317 focused on the target also affects the optical axis downstream of the EUV light produced. In particular, the optical axis O of the generated EUV radiation beam will typically be in the same direction as the optical axis 224 of the laser beam focused on the target droplet, as is evident from the arrangement shown for example in FIG. 2 . . As typically shown in FIG. 3 , a laser beam focused on a droplet target, typically a fuel target, is directed at the center of a collector mirror, eg mirror 380 shown in FIG. 3 or mirror CO shown in FIG. 2 . Irradiating the target through the aperture, the mirror focuses the generated EUV radiation along substantially the same optical axis as the laser beam focused on the fuel target. As such, the laser focusing system according to the invention enables the generation of a beam of EUV radiation along a substantially horizontal axis, whereas in known EUV radiation source designs for use in lithographic apparatus, the optical axis O of EUV radiation, e.g. For example, the angle of the EUV radiation beam applied to the illumination system IL shown in FIG. 2 is at a relatively large angle with respect to the horizontal plane. It has been observed by the inventors that the application of a relatively large angle adversely affects the design options for the illumination system IL and the projection system PS downstream. The relatively small angular application of the generated EUV radiation beam is designed to enable the design of optical systems downstream of EUV radiation sources with fewer components. As an alternative to the arrangement as shown in FIG. 3 , in which the second reflected laser beam 317 has an optical axis 370 substantially parallel to the horizontal plane, the laser focusing system can The angle between the optical axis 370 and the horizontal plane may be configured to be between 50 and 75 degrees.

It may be further pointed out that the laser focusing system according to the invention also results in an improved optical design compared to conventional or known designs of laser focusing systems as adapted for use in EUV radiation sources. As can be seen for example in FIG. 3 , the laser focusing system according to the present invention provides a curved mirror 330.2 as the last optical element to direct the laser beam 317 onto a fuel target or towards a target location 340 . apply In known laser focusing systems, typically a flat mirror will be applied as the last optical element. An example of such a known arrangement is schematically shown in FIG. 4 . A laser focusing system 400 as schematically shown in FIG. 4 is a curved mirror 400.1 configured to receive a laser beam 401 and reflect the laser beam 401 towards a second curved mirror 400.2. wherein the second curved mirror reflects the laser beam towards a third flat mirror 400.3, and the third flat mirror 400.3 focuses the laser beam along a substantially perpendicular optical axis 403 ( 402) to focus. The inventors have observed that known designs may suffer from one or more of the following drawbacks: Due to the deformation of the mirror, the flat third mirror 400.3 may lead to a significant contribution to aberrations of the system, such as astigmatism. there is. Since the angle of incidence on the flat third mirror depends on the position of incidence, this can also cause retardation. Also, because the flat third mirror is relatively close to the focal point 402 and thus the plasma generation position and its orientation are nearly horizontal, it may be more susceptible to contamination or degradation, such as degradation of coatings or optical surfaces.

According to the present invention, there is provided a laser focusing system in which this disadvantage is eliminated or mitigated. In an embodiment of the present invention, the last mirror 330.2 that focuses the laser beam 317 on the target or target location is thus a curved mirror rather than a flat mirror. Examples of such a mirror that can be applied to the present invention include a spherical mirror, a parabolic or parabolic mirror, an elliptical mirror, an axisymmetric mirror or a freeform mirror. According to the present invention, the second-to-last mirror 330.1 along the optical path of the laser beam focused on the target position is also a curved mirror. In an embodiment, the mirror is configured to diverge an received laser beam, ie a laser beam received from a laser source or beam delivery system. By applying the diverging mirror as the second-to-last mirror that diverges the received laser beam, the laser beam 317 focused at the target location has a relatively large numerical aperture NA while at the same time being relatively far away from the target location 340 . It can be configured to Or put differently, if the mirror 330.1 is a flat mirror rather than a diverging mirror, the cross-section of the beam 316 impinging on the second mirror 330.2 will be smaller, and also the focused laser beam 317 with a relatively large NA. ), the mirror 330.2 would have to be relatively close to the target position 340 . Having the second mirror 330.2 at a relatively large distance from the target location 340 provides the advantage of mitigating the risk of contamination of the mirror, where the contamination, for example, is when the focused laser beam 317 strikes the fuel target. occurs because As such, a sequence with a second-to-last diverging mirror 330.1 followed by a converging mirror 330.2 that reflects the focused laser beam towards the target location is considered to provide substantial advantages over prior art laser focusing systems. can be Additional advantages may be obtained by providing a focused laser beam towards a target location along a substantially horizontal optical axis.

In an embodiment of the present invention, the first and/or second curved mirrors of the laser focusing system are configured to receive the laser beam at a relatively small angle of incidence. In an embodiment, the angle of incidence of the laser beam to either the first curved mirror, the second curved mirror or both is 30 degrees or less, preferably 25 degrees or less. By doing so, a more accurate wavefront of the laser beam impinging on the droplet target or the fuel target can be realized. To realize this, the vertical positions of the mirrors must be relatively close to each other. In other words, when the vertical positions of the curved mirrors 330.1 and 330.2 are substantially different, it may be difficult to achieve a desired low angle of incidence. Typically, the vertical position of the laser beam 315 as received from the laser source or beam delivery system is lower than the desired vertical position of the focused laser beam 317 focused at the target position 340 . Also, for practical reasons, it may be cumbersome or undesirable to raise the laser source or laser beam delivery system to a desired level in order for the laser beam to reach a desired vertical position so that a relatively small angle of incidence of the curved mirror of the laser focusing system is realized. can To avoid this, embodiments of the present invention further comprise one or more additional mirrors configured to redirect a laser beam, eg, a laser beam 315 as received from the beam delivery system, towards the first curved mirror and there is. The one or more additional mirrors may also be considered part of a beam delivery system as the one or more additional mirrors are configured to redirect or reposition the laser beam such that the laser beam arrives at an appropriate position and orientation with respect to the first curved mirror. can

In an embodiment of the present invention, the amount of additional mirrors configured to redirect the laser beam towards the first curved mirror is less than five.

In embodiments of the present invention, the one or more additional mirrors may comprise or form a periscope or telescope system. Such a system may be configured to translate a laser beam 315 as provided in a vertical direction, for example by a laser source or beam delivery system. Such a periscope system as applied in an embodiment of the present invention thus makes it possible to translate the laser beam 315 received by the laser focusing system in a vertical direction.

5 schematically shows a laser focusing system 430 according to an embodiment of the present invention, wherein the laser focusing system includes a periscope system including a pair of mirrors 410.1, 410,2, This pair of mirrors, for example, align an incoming laser beam 315 received from a laser source or laser beam delivery system, and a reflected laser beam 318 from a second mirror 410.2 of the periscope system, raised vertically. and is configured to reflect in a manner that propagates along a substantially horizontal direction in position. In the embodiment as shown, the incoming laser beam 315 is raised vertically over a distance D, for example. By raising the laser beam 315 as received from a laser source or beam delivery system, the angle of incidence of the laser beam at the curved mirrors 330.1, 330.2 can be kept relatively small. This can be appreciated considering that without the flat mirrors 410.1 and 410.2, the mirror 330.1 must be in the position of the mirror 410.1 to receive the laser beam 315.

In the embodiment as shown, the periscope systems 410.1 and 410.2 include two planar mirrors 410.1 and 410.2. Alternatively, one or more mirrors of the periscope systems 410.1 and 410.2 may be curved mirrors. By applying one or more curved mirrors to the periscope system, the properties of the incoming laser beam 315 can be tuned. In particular, the dimensions of the incoming laser beam 315 may be controlled using one or more curved mirrors within the periscope system. Also, by using one or more curved mirrors in the periscope system, the outgoing laser beam 318 of the periscope system can be a diverging or converging laser beam.

As will be appreciated by those skilled in the art, a periscope system as applied to the present invention may also include two or more, for example three or four mirrors.

In an embodiment of the present invention, the laser focusing is a control configured to control the position of the target position 340 , ie the position of the focus of the second reflected laser beam 317 inside the EUV vessel 360 as shown in FIG. 3 . more units. This control of the focal position of the laser beam 317 facilitates ensuring that the laser beam 317 strikes a fuel target, eg, a tin droplet.

6 schematically shows the trajectory of a fuel target as it may occur within an EUV vessel. 6 schematically presents a fuel target source 510 , also referred to above as a droplet generator 226 , which is configured to generate a fuel target, eg, a stream of tin droplets. The stream of fuel target as generated is configured to propagate along axis 520 . The axis 520 intersects the focal point 530 of a collector mirror, for example mirror CO shown in FIG. 2 or mirror 380 shown in FIG. 3 . Axis 520 is considered to define the z-direction of the coordinate system as applied to the EUV vessel. In FIG. 6 , dotted line 550 represents the xy-plane inside the EUV vessel, where xy-plane 550 includes a focal point 530 . The focus 530 as shown corresponds to the target position 340 as shown in FIG. 5 , ie the position at which the laser beam 317 should be focused to realize effective conversion of the fuel target to EUV light. can be considered In the embodiment as shown, focus 530 may be, for example, at the origin of a defined xyz coordinate system. Due to imperfections in the fuel target source 510 and/or the fuel applied, the actual trajectory 522 of the fuel target may deviate somewhat from the ideal or desired trajectory 520 . Consequently, in order to effectively convert the fuel target into EUV light, the laser beam 317 as provided by the laser focusing system will have to be focused at a slightly different location 530'. In order to predict the possible departure trajectory of the fuel target, it is desirable to control the position of the target position, ie, the position at which the laser beam output by the laser focusing system is focused.

In order to control the position of the target position, the embodiment of the laser focusing system according to the present invention further comprises a control system 540 . The control unit 540 may be implemented as, for example, a controller, a microcontroller, a computer, or the like. The control unit 540 includes one or more input terminals 540.1 for receiving one or more input signals 542 and one or more output terminals 540.2 for outputting one or more output signals 544 for controlling the laser focusing system. may include

In an embodiment of the present invention, the control unit 540 as applied may be configured to receive as input signal 542 a target position signal indicative of the position of the fuel target at which the laser beam 317 of the laser focusing system will strike. there is. This target position signal may be generated by a detector configured to detect the position of a target, for example a fuel target. 6 , such a detector 570 is schematically shown. In the embodiment as shown, the detector 570 is configured to detect a target 532 , eg, a liquid fuel target 532 , when the target 532 traverses a plane 560 . In an embodiment, the detector may include multiple detectors or one or more detector arrays configured to determine the x, y position of the target 532 when the target traverses the plane 560 . Detector 570 may be further configured to output a target position signal indicative of the x, y position of the target. In the embodiment as shown, plane 560 is substantially parallel to plane 550 and is arranged between fuel target source 510 and plane 550 containing focal point 530 of the collector mirror. As such, the detector 570 will detect the fuel target 532 before the target target traverses the plane 550 . During the time the target moves from position 532 to position 530', the control unit 540 controls the coordinates of the position 530' based on the target position signal as received, ie the fuel target is in the plane 550 It is possible to derive a position that crosses Based on the determined coordinates of the position of the fuel target, the control unit 540 is then configured to control the laser focusing system in such a way that the focus of the laser beam 317 will substantially coincide with the position 530' of the fuel target. , may determine one or more output signals 544 , also referred to as control signals.

In embodiments, one or more output signals 544 may be applied, for example, to control the position, orientation and/or shape of one or more optical elements of the laser focusing system, whereby the laser output by the laser focusing system may be applied. Controls the position of the focal point of the beam, for example the laser beam 317 . In such embodiments, one or more output signals may be applied to control the position, orientation and/or shape of one or more mirrors, for example as applied in a laser focusing system according to the present invention. By controlling the position, orientation and/or shape of one or more mirrors as applied to the laser focusing system, the focal position of the second reflected laser beam 317 may be controlled in one or more degrees of freedom.

In embodiments, one or more optical elements of the laser focusing system may be mounted to one or more frames. In such embodiments, the position, orientation and/or shape of one or more mirrors as applied may be controlled by controlling the position of one or more frames. In embodiments having at least two optical elements of the laser focusing system, the at least two optical elements may be mounted to a common frame such that their position, orientation and/or shape may be controlled synchronously and/or simultaneously.

According to the present invention, it can be implemented to ensure that when the fuel target reaches the target position, the second reflected laser beam 317 as output by the laser focusing system is focused on the target position.

The strategy may include controlling the position, orientation or shape of at least one of a first periscope mirror, a second periscope mirror, a first curved mirror or a second curved mirror, or any combination thereof, thereby providing EUV with one or more degrees of freedom. This may include controlling the position of the focal point within the vessel. The control strategy as applied may further include controlling the timing of the laser beam or laser pulse, thereby controlling where along the trajectory of the fuel target the fuel target is irradiated.

As a first example, by controlling the position, orientation and shape of one of a first periscope mirror, a second periscope mirror, a first curved mirror, a second curved mirror, or any combination thereof, the control unit is configured to control the EUV It is configured to control the position of the focal point inside the vessel in 3 degrees of freedom. An example of this is schematically illustrated in FIG. 7 . 7 schematically shows a laser focusing system 530 according to an embodiment of the present invention, wherein the laser focusing system 530 includes a first periscope mirror 410.1, a second periscope mirror 410.2, and a first a curved mirror 330.1 and a second curved mirror 330.2. The laser focusing system 530 can advantageously be applied to focus the laser beam 317 to a focal point 620 , which for example corresponds to the focal point of the collector mirror of the EUV radiation source. In the example as shown, the Z-axis is assumed to coincide with the optical axis of the second reflected laser beam 317 as output by the laser focusing system 530 , the Y-axis is as indicated in the figure, and X The -axis is assumed to be perpendicular to the YZ-plane. Compared to the coordinate system of FIG. 6 , the X-axis as shown in FIG. 7 may for example coincide with or parallel to the Z-axis shown in FIG. 6 . In the embodiment as shown, the second curved mirror 330.2 is configured to be displaceable along the Z-axis, the Y-axis and the X-axis. Such displacement may be realized, for example, by actuator assembly 610 . Such an actuator assembly may include, for example, one or more actuators for displacing mirror 330.2 along an axis indicated. As will be appreciated by one of ordinary skill in the art, displacement of the mirror 330.2 in one of the Z-direction, Y-direction, or X-direction will also cause displacement of the focus 620 of the second reflected laser beam 317 . will be. As such, by controlling the position of the second curved mirror 330.2 in three degrees of freedom, the focus 620 can be adjusted according to the expected target position, ie the position where the fuel target is expected to be when the laser pulse is supplied. When using such an embodiment, for example, the control unit of the laser focusing system may be configured to determine where the fuel target is at a particular moment, for example the moment the laser is fired. In such an embodiment, a control unit, eg, thus, control unit 540 may determine a trajectory of the fuel target based on the received target position signal and use the trajectory to direct the laser beam 317 to the target. The laser focusing system can be controlled to focus the position. In such an embodiment, the timing or firing of the laser is thus assumed to be fixed. Based on the known moment at which the laser will be fired and the known trajectory of the target, the control unit determines where the target will be at the known moment of firing. In an alternative embodiment, the firing of the laser, in particular the timing of firing of the laser, may be considered a variable. In such an embodiment, it may be sufficient to control the focal position in only two degrees of freedom. Referring to FIG. 6 , it may be pointed out that the control unit 540 may be configured to determine when a particular detected target reaches or traverses the plane 550 when passing through the plane 560 . Based on this, the control unit may be configured to control the time to fire the laser beam 317 such that the laser is fired when the fuel target reaches or traverses the plane 550 . In this embodiment, it is therefore not necessary to control the position of the focal point along the Z-axis shown in FIG. 6 by controlling the orientation or position of the laser beam along the Z-axis. As such, in an embodiment of the present invention, the control unit of the laser focusing system is configured to control the position of the laser focus in two degrees of freedom combined with the controlled timing of the laser beam. In this embodiment, to ensure that the fuel target is irradiated by the laser beam 317 at a desired position, for example as close as possible to the focus of the collector mirror of the EUV radiation source in which the laser focusing system is used, The actuator assembly 610 as schematically shown may be configured to control the position of the focal point of the laser beam 317 in the YZ-plane as indicated, in combination with control of the timing of the laser beam.

7 , to control the position of the focus 620 of the laser beam 317, the actuator assembly 610 is configured to control the position of the second curved mirror 330.2 in 3 or 2 degrees of freedom. Consists of. A similar effect can be obtained by controlling the position of one or more other mirrors as applied to the laser focusing system according to the present invention.

In a second example of a control strategy as may be applied to the laser focusing system according to the present invention, the necessary focus control is distributed over a number of mirrors. This embodiment is schematically illustrated in FIG. 8 . 8 schematically shows a laser focusing system 630 according to an embodiment of the present invention, wherein the laser focusing system 630 includes a first periscope mirror 410.1, a second periscope mirror 410.2, and a first a curved mirror 330.1 and a second curved mirror 330.2. A similar coordinate system XYZ as shown in FIG. 7 is assumed. In the embodiment as shown, the laser focusing system 630 is configured to position or displace the second curved mirror 330.2 in the Z-direction, ie in the optical axis direction of the second reflected laser beam 317 . One actuator assembly 610 is further included. The laser focusing system 630 further includes a second actuator assembly 720 configured to rotate or tilt the second periscope mirror 410.2 about the Y-axis and about the X-axis. Thus, by means of the first actuator assembly 710 and the second actuator assembly 720 , the laser beam propagating through the laser focusing system is controlled in three degrees of freedom. By doing so, by controlling the position of the second curved mirror 330.2 in the Z-direction and tilting the second periscope mirror 410.2 about the X-axis and about the Y-axis (Z, Rx and Ry (Rx) and Ry represents rotation or tilting about the X and Y axes) The focus 620 of the second reflected laser beam 317 is thus controlled in three degrees of freedom: X, Y, Z. By doing so, the second reflection The focus 620 of the laser beam 317 can also be controlled in three degrees of freedom, so this laser focusing system 630 can be advantageously applied to control the position of the laser focus 620 inside the EUV radiation source.

In a manner similar to that discussed above, the control strategy as exemplified by the second example may also be combined with control of the firing timing of the laser beam. By doing so, control of the position or orientation in only two degrees of freedom may be sufficient to obtain effective control of the laser focus, ie, the control configured to focus the second reflected laser beam 317 at the target position of the fuel target.

9 schematically shows a third example of a control strategy as may be applied to an embodiment of the laser focusing system according to the present invention. 9 schematically shows a laser focusing system 930 according to an embodiment of the present invention, wherein the laser focusing system 930 includes a first periscope mirror 410.1, a second periscope mirror 510.2, and a first a curved mirror 330.1 and a second curved mirror 330.2. In the embodiment as shown, the second periscope mirror 510.2 is a curved mirror rather than a flat mirror. The second periscope mirror 510.2 may be a concave mirror or a convex mirror. In the embodiment as shown, the laser focusing system 930 further includes an actuator assembly 800 configured to control the position and orientation of the second periscope mirror 510.2. In particular, the actuator assembly 800 is configured to position or displace the second periscope mirror 510.2 along the optical axis 510.3 of the mirror 510.2. By doing so, the position of the focus 620 of the laser beam 317 along the Z-axis may be modified or adjusted. The actuator assembly 800 is further configured to rotate or tilt the second periscope mirror 510.2 about the Y-axis and about the X-axis, eg, in a manner similar to the actuator assembly 720 of the second example. can be By doing so, control of the position of the focal point 620 of the laser beam 317 in three degrees of freedom is again obtained. In a manner similar to that discussed above, a control strategy as exemplified by the third example may also be combined with control of the firing timing of the laser beam. By doing so, control of the position or orientation in only two degrees of freedom may be sufficient to obtain effective control of the laser focus, ie, the control configured to focus the second reflected laser beam 317 at the target position of the fuel target.

As a fourth example of a method for controlling the focal position using the laser focusing system according to the present invention, the use of one or more deformable mirrors may be mentioned. By using a deforming mirror, the position of the focal point 620 of the second reflected laser beam 317 can also be controlled. As an example, by adjusting the curvature of this deformable mirror, the focal position 620 can be displaced along the optical axis of the laser beam 317 .

In embodiments, various control strategies as described above may be advantageously combined.

In an embodiment of the invention, the control of the position of the focus of the second reflected laser beam 317 in a particular degree of freedom or direction is achieved by combined control of two or more optical components of the laser focusing system, for example mirrors. is set As will be appreciated by one of ordinary skill in the art, a variety of different optical components, for example mirrors as applied in laser focusing systems, may have different properties, such as different weights, different resonant frequencies, and the like. As a result, the control performance or ability with respect to the achievable accuracy or resolution may be different for different mirrors or optical components applied. Also, the available or obtainable range may differ for different optical components, in which respect the range represents the available or possible displacement range of the focus in a particular degree of freedom.

In view of these characteristics, it may be suboptimal to use one component to control the position of the focus with specific degrees of freedom. It may be advantageous to control the position of the focus in a particular degree of freedom, for example by combined control of said degrees of freedom using two or more optical elements.

Referring to the example as described above, the positioning of the focus of the laser beam 317 in the Z-direction as indicated in FIGS. It may be pointed out that it can be set by a translation of the periscope mirror 410.2 or a displacement or deformation of the curved periscope mirror 510.2. Rather than just using one of the above options to displace the focus of the laser beam 317 in the Z-direction, for example by a combined displacement or deformation of at least two components (eg target position It may be advantageous to realize the required displacement of the focus (based on the signal). By doing so, the requirement regarding controlling the position of the focus with a specific degree of freedom can be obtained by a combined effort of at least two components.

By way of example, control of the focal position of the laser beam 317 in the Z-direction may be obtained, for example, by controlling the displacement of the first mirror of the laser focusing system and by controlling the displacement of the second mirror of the laser focusing system. can In such an embodiment, the displacement of the first mirror may enable, for example, a displacement of the focal position in the Z-direction over a relatively large range, whereas the displacement of the second mirror, for example, over a relatively small range allows displacement of the focal position in the Z-direction over , but with greater accuracy. In this embodiment, the displacement of the first mirror can thus result in a coarse positioning of the focus in the Z-direction over a relatively large range, whereas the displacement of the second mirror thus over a relatively small range in the Z-direction may cause fine positioning of the focus to Using such cooperation of two or more optical components, improved performance may be realized and/or control requirements of one or more optical components may be relaxed.

In this embodiment according to the invention, ie in which the position of the focus is controlled with a certain degree of freedom by the cooperation of at least two components, the control unit of the laser focusing system, based on the target position signal as received, at least and determine a first control signal for controlling a first optical component of the two components and a second control signal for controlling a second optical component of the at least two components. In such an embodiment, the first control signal may be configured to displace, rotate or deform the first optical component to effectuate a first displacement of focus in a particular degree of freedom, while the second control signal may be configured to displace, rotate or deform the first optical component in a particular degree of freedom. be configured to displace, rotate, or deform the second optical component to effect a second displacement of focus; The combination of the first displacement and the second displacement results in the focus being displaced to the desired position.

Various control strategies may be applied to realize this control cooperation, in which two or more optical components, such as mirrors, are applied to realize a combined goal, for example, realizing a particular focal position with a particular degree of freedom.

In an embodiment, the control unit is configured to control the position of the focal point in one degree of freedom by controlling a single mirror in one degree of freedom or multiple degrees of freedom. Alternatively, the control unit may be configured to control the position of the focal point in one degree of freedom by controlling multiple mirrors in one or multiple degrees of freedom. In general, the control unit as applied in the embodiment of the laser focusing system according to the present invention may be configured to control the position of the focal point in N degrees of freedom by controlling one or more mirrors in M degrees of freedom, where M=N , and M and N are non-zero integers. Alternatively, the control unit may be configured to control the position of the focal point in N degrees of freedom by controlling one or more mirrors in M degrees of freedom, where M≠N.

In an embodiment of the invention, a set of mirrors as applied, namely a first curved mirror, a second curved mirror and optionally one or more additional mirrors, comprises at least one long range mirror and at least one stage It includes a short range mirror. Within the meaning of the present invention, a long range mirror refers to a mirror that can be displaced over a relatively wide range, whereas a short range mirror refers to a mirror that can be displaced over a relatively short range. As such, in an embodiment of the present invention, the control unit of the laser focusing system is configured to control the position and/or orientation of the at least one long range mirror over a relatively large range and over a relatively short range and at least one short range mirror. may be configured to control the position and/or orientation of

In such an embodiment, the control unit of the laser focusing system may include, for example, a low-bandwidth controller for controlling the at least one long-range mirror and a high-bandwidth controller for controlling the at least one short-range mirror. . The low-bandwidth controller may, for example, be configured to apply a setting and forget control of the long-range mirror. In such an embodiment, the low-bandwidth control may be used to desaturate the high-bandwidth control.

In an embodiment, the at least one long range mirror may be controlled at a bandwidth of less than 5 Hz. In an embodiment, the at least one short range mirror may be controlled at a bandwidth greater than 0.1 Hz.

In order to displace or deform one or more mirrors of the laser focusing system according to the present invention, various types of actuators may be applied. Examples of such actuators include, but are not limited to, electromagnetic actuators such as Lorentz actuators or piezoelectric actuators. In general, any linear or rotary actuator may be adapted or adapted to be applied to control the position, orientation or shape of a mirror as applied to a laser focusing system according to the present invention.

5 to 9 illustrate various aspects and embodiments of a laser focusing system according to the present invention. As mentioned, the laser focusing system uses a curved mirror as the final and second-to-last mirrors to direct the laser beam towards a focal or target location 340 , 530 , 620 . Various embodiments of the laser focusing system according to the present invention include additional mirrors upstream of the last and second-to-last curved mirrors.

It may be pointed out that various options exist for the position and orientation in space of the different mirrors as applied. In this regard, it may be pointed out that although the laser beams shown in FIGS. 5 to 9 are drawn in a single plane, this need not be the case in practice.

Figures 10 to 12 thus schematically show three possible embodiments of the laser focusing system according to the invention in which all laser beams are not arranged in the same plane. Such an arrangement may also be referred to as an out-of-plane arrangement.

FIG. 10 schematically shows the path of a laser beam 1000 received by a first mirror 1010 and then traveling along mirrors 1020 , 1030 , 1040 towards a focal point FP. In the embodiment as shown, mirrors 1040 and 1030 correspond to the last and second-to-last mirrors 330.2 and 330.1, respectively, for example as described above.

In the embodiment as shown, mirrors 1010 and 1020 may correspond to, for example, mirror or periscope systems such as mirrors 410.1 and 410.2 described above.

In the embodiment as schematically shown in FIG. 10 , the laser beam 1000 as received by the first mirror 1010 and the laser beams 1000.1 and 1000.2 directed to the mirrors 1020 and 1030 respectively are planar, That is, it can be observed that the cube (cube) 1050 is arranged in the front plane. In the embodiment as shown, the laser beams 1000.2, 1000.3, 1000.4 are also arranged in the plane indicated by reference numeral 1060. In the embodiment as shown, the laser beams 1000 , 1000.1 and 1000.2 are thus arranged in a first plane, whereas the laser beams 1000.2 , 1000.3 and 1000.4 are arranged in a different second plane. It can be seen that in the embodiment as shown, the first plane is perpendicular to the second plane. It may be pointed out that this is not necessary.

11 schematically shows an embodiment of a laser focusing system according to the invention, wherein the first plane, ie the front plane of the cube 1050 comprising the laser beams 1000 , 1000.1 and 1000.2 , is the second plane ( Not perpendicular to 1070 , the second plane contains laser beams 1000.2 , 1000.3 and 1000.4 . It can also be pointed out that the laser beam 1000.1 is no longer perpendicular to the laser beam 1000 , compared to the arrangement as schematically shown in FIG. 10 .

10 and 11 , laser beams 1000.2 , 1000.3 and 1000.4 are arranged in the same plane, either plane 1060 or plane 1070 . It may be pointed out that alternative out-of-plane arrangements may be devised that do not require each plane to contain three of the laser beams 1000, 1000.1, 1000.2, 1000.3 and 1000.4.

12 schematically shows an alternative embodiment of a laser focusing system according to the invention, wherein the laser beams 1000.2, 1000.3, 1000.4 are not arranged in the same plane. In the embodiment as shown, laser beams 1000 , 1000.1 and 1000.2 are arranged in the front plane of cube 1050 , while laser beams 1000.3 and 1000.4 are arranged in plane 1060 , there is. In contrast to the arrangement of Figure 10, laser beam 1000.2 is not arranged in the same plane as laser beams 1000.3 and 1000.4.

From an optical point of view, it may be desirable to have the laser beam reaching the second to last mirror, the laser beam reaching the final mirror, and the laser beam emitted by the final mirror being in the same plane.

Although the mirrors 1010, 1020 as shown in Figures 10-12 are referred to as a periscope system, it may be pointed out that the present invention may also be practiced without the use of a periscope system.

In this regard, a single mirror configured to receive a laser beam, such as laser beam 1000 , and to redirect laser beam 1000 toward a second-to-last curved mirror 1030 is used in FIGS. 10-12 . It may be pointed out that it is possible to replace mirrors 1010 and 1020 as shown in .

In general, the laser focusing system according to the present invention is thus, in addition to the last and second-to-last curved mirrors, one or more additions for redirecting the laser beam towards the first curved mirror, ie the second-to-last curved mirror. It may include a negative mirror. In such an arrangement, a plane can be defined comprising a laser beam received by a most downstream mirror of said at least one further mirror and a laser beam reflected by said downstream mirror towards a second-to-last curved mirror. . 10-12 , mirror 1020 may serve as the downstream one of one or more additional mirrors.

Referring to the use of the one or more additional mirrors above, the out-of-plane arrangement of the laser focusing system may also include a second-to-last curved mirror, e.g., mirror 1030, by way of a downstream mirror, e.g., mirror 1020. Defined as an arrangement in which the laser beam as reflected towards it, the laser beam reflected by the second to last curved mirror, and the laser beam reflected by the final curved, eg mirror 1040, are not coplanar can be In this arrangement, the laser beam directed towards the first curved mirror, the first reflected laser beam and the second reflected laser beam are thus arranged in substantially different planes.

The laser focusing system according to the present invention can be advantageously applied to the laser source according to the present invention. Such a laser source may include, for example, a seed laser, one or more power amplifiers and a selective beam delivery system.

The laser source according to the invention can advantageously be applied to an LPP radiation source, for example a radiation source for generating EUV radiation suitable for use in an EUV lithographic apparatus.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The behavior of the device is a computer program comprising a sequence of one or more machine readable instructions for implementing specific steps of the method as disclosed above, or a data storage medium having such a computer program stored therein (eg, a semiconductor memory, magnetic or optical disks). The above description is intended to be illustrative and not restrictive. Accordingly, it will be apparent to those skilled in the art that modifications may be made to the invention as set forth without departing from the scope of the claims set forth below.

Claims (59)

A laser focusing system for use in an EUV radiation source, comprising:
- a first curved mirror configured to receive the laser beam from the beam delivery system and to generate a first reflected laser beam;
- a second curved mirror configured to receive the first reflected laser beam and to generate a second reflected laser beam;
wherein the laser focusing system is configured to focus the second reflected laser beam to a target location within the vessel of the EUV radiation source. The laser focusing system according to claim 1, wherein the angle between the optical axis of the second reflected laser beam and the horizontal plane is less than 25 degrees, preferably less than 20 degrees. The laser focusing system of claim 1 , wherein the angle between the optical axis of the second reflected laser beam and the horizontal plane is between 50 degrees and 75 degrees. 4. The laser focusing system of claim 1, 2 or 3, wherein the first curved mirror is configured to receive the laser beam at an angle of incidence less than 30 degrees. 4. The laser focusing system of claim 1, 2 or 3, wherein the first curved mirror is configured to receive the laser beam at an angle of incidence that is between 30 and 45 degrees. 6 . The laser focusing system of claim 1 , wherein the second curved mirror is configured to receive the laser beam at an angle of incidence less than 30 degrees. 6. The laser focusing system of any preceding claim, wherein the second curved mirror is configured to receive the laser beam at an angle of incidence that is between 30 and 45 degrees. 8. The laser focusing system of any preceding claim, wherein the first curved mirror comprises a parabolic, spherical, elliptical, axisymmetric or freeform reflective surface. 9. The laser focusing system of any preceding claim, wherein the second curved mirror comprises an elliptical, parabolic, spherical, axisymmetric or freeform reflective surface. 10. The laser focusing system of any preceding claim, further comprising one or more additional mirrors configured to redirect the laser beam towards the first curved mirror. 11. The laser focusing system of claim 10, wherein the amount of additional mirrors configured to redirect the laser beam towards the first curved mirror is less than five. 11. The laser focusing system of claim 10, wherein the one or more additional mirrors are configured to overcome an orientation difference between the incident and the first curved mirror requested by the laser beam. 11. The laser focusing system of claim 10, wherein the one or more additional mirrors comprises a periscope system. 14. The laser focusing system of claim 13, wherein the periscope system is configured to overcome a position difference between the received laser beam and the redirected laser beam. 15. The laser focusing system of claim 13 or 14, wherein the periscope system is configured to translate the laser beam in a substantially vertical direction. 16. The periscope system according to any one of claims 13 to 15, wherein the periscope system includes a first periscope mirror for receiving the laser beam and reflecting the laser beam towards a second periscope mirror, the second periscope mirror and the periscope mirror is configured to reflect the laser beam towards the first curved mirror. 17. The system of any of claims 10-16, wherein at least one of the additional mirrors is curved. 18. The laser focusing system of claim 17, wherein the at least one additional curved mirror comprises an elliptical, parabolic, spherical, axisymmetric or freeform reflective surface. 19. The laser focusing system of claim 17 or 18, wherein the one or more curved additional mirrors form a telescopic system configured to vary the diameter of the laser beam. 20. The laser focusing system of claim 18 or 19, wherein the one or more curved additional mirrors form a telescopic system configured to alter the divergence of the laser beam. 21. A laser according to any one of claims 10 to 20, wherein the laser beam directed towards the first curved mirror, the first reflected laser beam and the second reflected laser beam are arranged substantially in the same plane. focusing system. 21. A focusing laser according to any one of claims 10 to 20, wherein the laser beam directed towards the first curved mirror, the first reflected laser beam and the second reflected laser beam are arranged in substantially different planes. system. 21. The laser beam according to any one of claims 10 to 20, wherein the laser beam received by the at least one additional mirror and the laser beam redirected towards the first curved mirror define a first plane, the The first reflected laser beam and the second reflected laser beam define a second plane, the second plane being different from the first plane. 24. The laser focusing system of claim 23, wherein the first plane is substantially perpendicular to the second plane. 24. The laser focusing system of claim 23, wherein the second plane is not perpendicular to the first plane. 26. The laser focusing system of claim 23 or 25, wherein the second plane is parallel to the first plane. 25. A laser focusing system according to claim 22, 23 or 24, wherein the laser beam directed towards the first curved mirror is arranged in both the first plane and the second plane. 25. A laser focusing system according to claim 22, 23 or 24, wherein the laser beam directed towards the first curved mirror is not arranged in a second plane. 29 . The laser focusing according to claim 1 , wherein the laser beam as received by the first curved mirror, the first reflected laser beam and the second reflected laser beam are not coplanar. system. 30. The laser focusing system of any preceding claim, further comprising a control unit configured to control a focal position of the second reflected laser beam inside the EUV vessel. 31. The method of claim 30, wherein the control unit is configured to receive a target position signal indicative of a position of a target with respect to the second reflected laser beam, and wherein the control unit is configured to position the second reflected laser beam based on the target position signal. A laser focusing system configured to control the position of the focal point. 32. The laser focusing system of claim 31, wherein the target position signal comprises a target position trajectory indicative of the position of the target as a function of time. 32. The laser focusing system of claim 31, wherein the control unit is configured to control the timing of the laser beam based on the target position trajectory. 34. The laser focusing system of claim 33, wherein the control unit is configured to control the position of a focal point inside the EUV vessel in a plane substantially perpendicular to the target position trajectory. 35. The laser focusing system of claim 34, wherein the control unit is configured to control the timing of the laser beam to focus the laser beam onto the target when the target reaches or traverses the plane. 36. The laser focusing system of claim 35, wherein said plane comprises a focal point of a collector mirror of said EUV radiation source. 37. The method of any one of claims 30 to 36, wherein the control unit controls the position, orientation or shape of at least one of the first curved mirror or the second curved mirror or the one or more additional mirrors. and to control the position of the focal point within the EUV vessel in three degrees of freedom. 38. The laser focusing system of claim 37, wherein the control unit is configured to control the position and/or orientation of the second curved mirror in one, two or three degrees of freedom. 39. The laser focusing system of claim 38, wherein the control unit is configured to control the position and/or orientation of the first curved mirror in one, two or three degrees of freedom. 40. The laser focusing system of claim 39, wherein the first curved mirror and the second curved mirror are mounted onto a frame. 41. The laser focusing system of claim 40, wherein the control unit is configured to control the position and/or orientation of the frame in one, two or three degrees of freedom. 42. The method of any of claims 38-41, wherein the first curved mirror, the second curved mirror and the one or more additional mirrors comprise at least one long range mirror and at least one A laser focusing system with a short range mirror. 43. The method of claim 42, wherein the control unit controls the position and/or orientation of the at least one long range mirror over a relatively large range and controls the location and/or orientation of the at least one short range mirror over a relatively short range. A laser focusing system configured to control. 44. The laser focusing system of claim 43, wherein the at least one long range mirror includes a set and forget function. 45. The laser focusing system of claim 44, wherein the at least one long range mirror comprises a bandwidth of less than 5 Hz. 44. The laser focusing system of claim 43, wherein one said at least one short range mirror comprises a bandwidth greater than 0.1 Hz. 47. A laser focusing system according to any one of claims 32 to 46 with reference to claim 31, wherein the control unit is configured to control the position of the focal point in one degree of freedom by controlling a single mirror in a single degree of freedom or multiple degrees of freedom. . 48. A laser focusing according to any one of claims 32 to 47, wherein the control unit is configured to control the position of the focal point in one degree of freedom by controlling a plurality of mirrors in one degree of freedom or multiple degrees of freedom. system. 49. The method according to any one of claims 32 to 48, wherein the control unit is configured to control the position of the focal point in N degrees of freedom by controlling one or more mirrors in M degrees of freedom. =N laser focusing system. 50. The method according to any one of claims 32 to 49, reciting claim 31, wherein the control unit is configured to control the position of the focal point in N degrees of freedom by controlling one or more mirrors in M degrees of freedom, wherein M A laser focusing system with ≠N. 51. The laser focusing system of any of claims 31-50, further comprising a detector configured to detect a position of the target, wherein the detector is configured to output the target position signal. 52. The laser focusing system according to any one of claims 31 to 51, wherein the control unit is configured to control the position of a focal point inside the EUV vessel in a plane substantially perpendicular to the trajectory of the target in two degrees of freedom. 53. The laser focusing system of any preceding claim, wherein the laser beam has an optical axis substantially parallel to the horizontal plane. 17. The laser focusing system of claim 16, wherein the first periscope mirror and/or the second periscope mirror are substantially flat mirrors. 17. The laser focusing system of claim 16, wherein the first periscope mirror and/or the second periscope mirror comprises a curved mirror. 17. The laser focusing system of claim 16, wherein the first periscope mirror and/or the second periscope mirror are substantially flat mirrors. 57. A laser source comprising a laser focusing system according to any of the preceding claims. An EUV radiation source comprising a laser source according to claim 57 . 59. A lithographic apparatus comprising an EUV radiation source according to claim 58.

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